A birefringent polarizer assembly is known from JP 2000-009932 A. An apparatus for inspecting semiconductor slabs of the type mentioned in the introduction is known from US 2013/0070331 A.
Without restricting the generality, such a polarizer assembly is used for spatially separating light beams having different polarizations. Light having different polarizations is used in particular in apparatuses for inspecting semiconductor slabs in order to obtain a better surface contrast when illuminating the slabs.
The polarizer assembly includes at least two prisms constructed of a birefringent material, wherein the principal crystal axes of the respective materials from which the prisms are constructed have an orientation with respect to one another that is characteristic of the polarizer assembly.
In crystal optics, the principal crystal axis denotes the direction in an optically anisotropic uniaxial crystal along which each polarization component of a light beam experiences the same refractive index. It goes without saying here that a light beam includes a plurality of light rays or partial beams of light rays. When the light beam is incident in a direction that it is not parallel to the principal crystal axis, the light beam is separated into a first partial beam, which is designated as ordinary ray, and into a second partial beam, which is designated as extraordinary ray. In principle, there is also the possibility of using multiaxial crystals having more than one principal crystal axis for the prisms of the polarizer assembly. Upon oblique incidence on an interface, the light beam is split into partial beams having different polarizations. In this case, the first partial beam has a first polarization state, and the second partial beam has a second polarization state, which in each case have linear polarizations without restricting the generality. The first and second polarization states here are typically represented as the eigenpolarizations, namely the S-polarization and the P-polarization, wherein the P-polarization is oriented in the plane of incidence and the S-polarization is oriented perpendicularly to the plane of incidence. It goes without saying, however, that polarization states can also be present which can have a different orientation of the polarization direction than the S- or P-polarization and can also have a different polarization than the linear polarization.
The polarizer assembly splits the light beam which is incident from a first half-space and which impinges on a first light entrance surface of a first prism along a principal light incidence direction into partial beams having different polarizations which propagate through the polarizer assembly and via a further light exit surface of a further prism into a second half-space. The principal light incidence direction should be understood to mean the optical axis of the polarizer assembly.
The spatial separation of the partial beams in the second half-space is in turn characteristic of the orientation of the principal crystal axes of the respective prisms used in the polarizer assembly with respect to one another.
Depending on the purpose of use, specific advantages and disadvantages arise here for the prism arrangement described in the introduction, which is also designated as a Wollaston prism arrangement, compared with other forms of the arrangement of prisms such as the Rochon arrangement, for example, in which the principal crystal axes of the respective prisms are aligned differently relative to one another and to the principal light incidence direction.
A specific advantage of the Wollaston prism arrangement here is that a splitting angle of the individual partial beams that is increased in relation to the Rochon arrangement is achieved in the second half-space.
However, the Wollaston prism arrangement has the disadvantage that both partial beams passing in the second half-space downstream of the prism arrangement have an orientation that is not parallel to the principal light incidence direction, which is disadvantageous particularly when the birefringent polarizer assembly is used in an imaging system. Furthermore, both partial beams passing in the second half-space have a beam offset in terms of location and in terms of angle in relation to the principal light incidence direction and a great spectral color dependence. Moreover, the Wollaston prism arrangement, in particular, when used in an apparatus for inspecting semiconductor slabs, which typically include light sources having high etendues, exhibits an inadequate spatial separation of the different partial beams having different polarizations. The disadvantages mentioned above therefore make it more difficult to implement a modular use of a Wollaston prism arrangement in an optical set-up, since here the installation or demounting of the Wollaston prism arrangement leads to a disadvantageous influencing of the beam characteristic.
The birefringent polarizer assembly known from the document JP 2000-009932 A cited in the introduction includes three prisms. The light exit surface of the last prism as viewed in the light propagation direction is inclined at an angle relative to the principal light incidence direction. In this case, the angle is set in such a way that the light beam emerging from the last light exit surface is parallel to the incident light beam.
WO 90/15357 A1 discloses a birefringent polarizer assembly in which a wholly or partly plane-parallel plate or a wholly or partly plane-parallel air space is present between the first and last optical elements of the polarizer. The light beams emerging from the polarizer are thereby intended to be oriented in a defined manner.
DE 22 17 175 A discloses a polarizer assembly which consists of two prisms, between which a wedge-shaped air gap is present.
U.S. Pat. No. 6,661,577 B1 discloses a beam splitter designed as a birefringent Wollaston polarizer assembly and provided for splitting a beam into partial beams having different polarizations. For this purpose, the polarizer assembly includes a Wollaston prism, which splits the two partial beams at a splitting angle relative to the principal light incidence direction. The two partial beams split relative to one another are refracted by a birefringent double wedge disposed downstream at a distance from the Wollaston prism in such a way that the partial beams disposed downstream of the double wedge are aligned parallel to the principal light incidence direction.
The known beam splitter has the disadvantage, however, that the use of different optical elements used for beam splitting and for correction of the beam direction in a spatially extended optical set-up is disadvantageous with regard to a compact design of the beam splitter having low susceptibility to disturbances. What is disadvantageous about the known beam splitter, moreover, is that the parallel alignment of both partial beams makes it more difficult to separate the different partial beams from one another since the latter no longer diverge relative to one another.
Furthermore, WO 2005/085917 A1 discloses a broadband Glen-Thompson polarizer for splitting a principal ray into two partial beams having different polarizations. In this case, one of the two partial beams is subjected to total internal reflection at the interface between the two prisms and is segregated from the partial beam having the other polarization.
What is disadvantageous about this arrangement is that only greatly birefringent materials can be used, which greatly restricts the flexibility in the choice of materials, particularly in the visible or infrared spectral range. Moreover, such an arrangement is not suitable without restriction for the ultraviolet spectral range since sufficiently great birefringence cannot be obtained on account of the dispersion of the materials typically used. At the interface at which one partial beam is segregated by total internal reflection, furthermore, on account of the very shallow angle of the interface between the two prisms in relation to the principal light incidence direction, very great reflection losses of the partial beam which passes through the entire beam splitter typically occur as well. This aspect is disadvantageous particularly with regard to a desired high transmission of the optical component.